[MIPS] Fix build error if CONFIG_KALLSYMS is undefined.

  CC      arch/mips/kernel/traps.o
arch/mips/kernel/traps.c: In function 'show_backtrace':
arch/mips/kernel/traps.c:110: warning: unused variable 'ra'

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>